Low noise line powered DAA with differential feedback

ABSTRACT

A telephone line interface or data access arrangement (DAA) is provided which draws power for the customer premises equipment from the telephone line. The DAA includes a shunt regulator as the power source drawing power from the telephone line, and a line modulator in series with the shunt regulator. A sense resistor is placed in series between the line modulator and shunt regulator to sense the line current. Accordingly, DC termination and AC modulation characteristics as presented by the DM to the telephone line can be adjusted by variation of an AC signal with DC offset input to the amplifier driving the line modulator without requiring a different DAA for use in regions or countries having various DC termination and AC modulation requirements. The differential feedback from a sense resistor reduces the noise induced in the circuit from the shunt regulator.

This application is similar to U.S. application No. 09/028,061, entitled“Low Noise Line Powered DAA With Feedback” by Hollenbach et al., whichis explicitly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a low noise telephone line interface for dataaccess arrangements (DAA). More particularly, it relates to a linepowered DAA having controllable DC termination and AC modulation.

2. Background of Related Art

Many portable computer devices utilize modems and other data devices forcommunicating over a telephone line. In such devices, battery size andweight is an important consideration. A balance must be reached betweenthe size and weight of the overall device, which is dictated largely bythe choice of battery, and an acceptable length of operation betweencharges.

Unfortunately, although operating an acceptable length of time whenrunning typical application programs, the modems and other datacommunication devices of a portable computer utilize a large amount ofpower when communicating over a telephone line. The battery source whichpowers both the portable computer device and its modem is typicallysized for general computing applications, and runs out of power quicklywhen actively communicating over a telephone line through a modem.Portable computer devices such as personal digital assistants (PDAs),hand held PCs (HPC), PCMCIA modems, and portable data terminals aredesigned to operate up to several hours on a single battery charge, butoperate only fractions of an hour on a single battery charge whencommunicating via modem. Thus, although portable computer devicesoperate a sufficient length of time for quick data transfers over amodem powered completely by a battery, they typically require thatexternal AC power be applied to allow for longer uses of the modem. Itis therefore desirable for battery powered computer devices including amodem to draw power, in addition to the inherent battery, from asecondary power source.

The DC power inherent in a telephone line provides a convenient sourceof power, but there are often limitations and restrictions which limitthe ability of a modem to derive power from the telephone line. Forinstance, present regulations in the United States are such thatsignificant current may only be drawn from the telephone line when thetelephone or modem is in an off-hook or active condition. In order tohold the telephone line in an off-hook condition, current in theapproximate range of 20 milliamps (mA) to 150 mA must be drawn. Thus,the maximum amount of current drawn from the telephone line is limited.

Modems which are designed to be powered entirely from the telephone lineare known, but these designs either suffer from an extremely constrainedpower budget, or are wasteful of the available current. Moreover, modemsin general are also subject to government constraints, e.g., FCC Part 68requirements for telephones in the U.S., and limitations on effects andnoise which may be placed back on the telephone line, placing furtherrestrictions on the use of power from the telephone line.

A Data Access Arrangement (DM) provides the physical interface between adata source such as a modem, and a telephone line. The DAA isresponsible for presenting the proper DC termination and AC modulationcharacteristics to the telephone line. For instance, the DAA must draw aminimum amount of DC current when in the off-hook condition to hold thetelephone line in an off-hook condition, but at the same time must drawno more than a maximum amount of current while in the off-hookcondition. Thus, the DC termination or load must be within prescribedlimits corresponding to the pertinent telephone standards of the countryin which the DAA is being used. Accordingly, the modem must operate withno more than the prescribed maximum current available from a telephoneline in the particular country in which the DAA is being used.

In today's global economy, it is desirable to design and manufactureproducts for use in any of a plurality of different countries, not justfor a single country. However, the regulations with respect to theavailable amount of power on a telephone line in the different countriesvaries greatly, as does the minimum amount of current the customerpremises equipment must draw (i.e., the DC load) to guarantee anoff-hook condition. Thus, a DAA used in one country must provide DCtermination and AC modulation characteristics to a telephone line whichmay be and often are entirely different from the DC termination and ACmodulation characteristics presented by a DAA used in another country.Accordingly, manufacturers conventionally manufacture different DAAsincluding different physical components for each of the varyingcountries.

FIG. 1 shows a typical DM including a parallel combination of an AC linemodulator 102 and a DC termination circuit 103. A modem or other datadevice including the DAA shown in FIG. 1 provides an AC modulationsignal 108 to drive the AC line modulator 102. When used in countrieswith differing standards, many of the components in the DC terminationcircuitry 103 and/or in the AC line modulator 102 are changed to conformto the telephone company standards in the particular country in whichthe DAA is being used. This unfortunately requires productsincorporating the DMs to be manufactured and packaged separately foreach of a plurality of varying countries.

There is a need for a DM which provides low noise line power for a modemor other data device, and which is software customizable for each of aplurality of varying countries without requiring different hardwarecomponents to be in conformance with the different DC termination and ACmodulation requirements of those particular countries.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, a telephoneline interface comprises a shunt regulator. A line modulator is inseries with the shunt regulator. An amplifier, powered by the shuntregulator, drives the line modulator. The telephone line interfacefurther includes a feedback loop from the line modulator to theamplifier.

In accordance with another aspect of the present invention, a telephoneline interface includes a shunt regulator to draw power from a telephoneline. A sense resistor is connected in series with the shunt regulator.A line modulator is in series with the sense resistor and the shuntregulator. A differential feedback path is also provided from oppositeterminals of the sense resistor to the line modulator.

A method of powering customer premises equipment from a telephone lineis also provided, comprising drawing power from the telephone line witha shunt regulator. A line modulator is provided in series with the shuntregulator. An amplifier is powered with power drawn from the telephoneline. The line modulator is driven with the line powered amplifier, anda signal is fed back from the line modulator to an input of the linepowered amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparent tothose skilled in the art from the following description with referenceto the drawings, in which:

FIG. 1 shows a conventional DAA.

FIG. 2 shows a possible configuration for a DM providing line power to amodem useful for explaining the present invention.

FIG. 3 shows another possible configuration for a DAA providing linepower to a modem useful for explaining the present invention.

FIG. 4 shows a line modulator and shunt regular in series in anarrangement useful for explaining the present invention.

FIG. 5 shows a low noise line powered DM in accordance with theprinciples of an embodiment of the present invention.

FIG. 6 is a more detailed schematic drawing of the low noise linepowered DM in accordance with the principles of an embodiment of thepresent invention.

FIG. 7 shows a line powered DAA in accordance with the principles ofanother embodiment of the present invention.

FIG. 8 is a more detailed schematic drawing of the line powered DAAshown in FIG. 7.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A driving motivation of the present invention was to provide a linepowered DM device having reduced noise which is otherwise present. Thepresent invention is particularly useful for use with modems or otherline powered data sources, including a digital signal processor (DSP)which provides a modulating AC signal for driving a line modulator.Although a DSP is not required to practice the present invention, theuse of a line powered DSP generally increases the noise of a DAA andthus makes the need for reduced noise techniques that much more acute.

A suitable conventional DAA circuit is shown in FIG. 1. However, this DMdoes not provide power from the telephone line. Nevertheless, thisconventional circuit typically exhibits low noise generation in the DAAbecause the tip polarity TP signal and ring polarity signal RP provide acommon mode to both the line modulator 102 and the DC terminationcircuit 103 which are in parallel across the tip polarity signal TP andring polarity signal RP.

The DC termination 103 also provides the important function ofprotecting the DAA, including the line modulator 102 and other modemcircuitry, from voltage surges on the telephone line in excess of, e.g.,30 volts. Conventional DC termination circuits 103 use bipolar circuitryin connection with the telephone line. Although MOS components makebetter switches, they are better suited for lower voltage applicationsas they are subject to latch up or other destructive abnormalities atvoltages as high as 30 volts. Thus, bipolar components are preferredover MOS components in conventional DC termination circuits 103.Moreover, a further disadvantage of the conventional circuit topology asshown in FIG. 1 is that the DC termination circuitry 103 is fixed andseparate from the AC modulation circuitry, and not otherwisecontrollable in characteristic without changing components. Thus, adifferent DC termination circuit 103 is required for each varyingcountry in which the DAA will be used.

One alternative arrangement to the prior art DAA shown in FIG. 1 adds afeature of providing line power to a line modulator, modem and/or othercircuitry. The circuit in FIG. 2 implements a shunt regulator 200 inparallel with each of a line modulator 102 and a DC termination circuit103 to provide power drawn from a telephone line to an amplifier 220 andother modem or data processing circuitry.

In FIG. 2, the shunt regulator 200 is placed across the tip polaritysignal TP and ring polarity signal RP, and thus is in common mode withthe circuitry it is protecting, i.e., the shunt regulator 200 and thepowered circuitry share a common ground reference, i.e., the ringpolarity signal RP. The conventional AC line modulator 102 is driven byan AC signal from an amplifier 220. The amplifier 220 and line modulator102 are both powered by power derived by the shunt regulator 200 fromthe telephone line. The shunt regulator 200 provides a constant voltagepower source, e.g., at 3 volts, at a maximum current level determinedonly by the draw of the amplifier 200 and other modem circuitry. Thetotal power drawn by the modem circuitry is represented by currentsource 222. Because the DC termination circuitry 103 is in parallel withthe shunt regulator 200, the DC termination circuitry 103 ensures thatthe voltage input to the shunt regulator 200 will not exceed apredetermined amount, e.g., 3 or 5 volts. Thus, MOS circuitry can beutilized in the shunt regulator 200 or other portions of the modem. Inan ideal situation, it is desired that the shunt regulator 200 merelyregulate power drawn by the modem, but not actually draw and thus wastecurrent itself. Generally, wasted current is to be avoided in linepowered devices.

The circuit of FIG. 2 retains the advantages the protection of aconventional parallel DC termination circuit 103 provides, e.g., 30 voltmaximum voltage protection, but does not allow easy adjustment of the DCtermination circuit 103 for use in various countries without changingcomponents within the DC termination circuit 103. As discussed, this isdisadvantageous in a world in which it is desirable to sell anidentically-manufactured product in more than one country.

In an alternative arrangement, the DC termination circuit couldtheoretically be combined with a line modulator as depicted in thecircuit shown in FIG. 3.

In FIG. 3, a combined line modulator and DC termination circuit 302 isformed from an operational amplifier (op amp) 340, bipolar transistor342, and resistor 344. The bipolar transistor 342 and resistor 344 areconnected in series across the tip polarity TP and ring polarity RPsignals from a telephone line. The op amp 340 receives the AC modulatingsignal at the non-inverting input, while the inverting input of the opamp 340 is connected to the emitter of bipolar transistor 342. The opamp 340 is powered from the line power derived by the shunt regulator200, as is the amplifier 220 and other circuitry providing the ACmodulating signal.

The circuit shown in FIG. 3 is suitably noise free because the combinedline modulator/DC termination circuit 302 and shunt regulator 200 have acommon ground reference, i.e., the ring polarity signal RP, but suffersfrom the possibility of wasted current, which is extremely valuable in aline powered device. To eliminate wasted current, it might betheoretically possible to determine the exact amount of current requiredby the line modulator/DC termination circuit 302 and other modemcircuitry which is to be powered by the shunt regulator 200. However,current usage typically varies based on a multitude of factors, e.g.,the manufactured tolerance of components, the frequency of the ACmodulating signal, the temperature of the circuitry, etc. Thus, a safetyfactor reserving an amount of available current must be provided by theshunt regulator 200 to ensure proper operation of the DM over aspecified range of temperature and frequency. This safety factorcorresponds to wasted and/or unusable current from the telephone line inan off-hook condition, further limiting the amount of power from atelephone line which can actually be used.

To maximize the use of line power in a DAA with reduced noise, thepresent invention places a combined line modulator/DC terminationcircuit in series with a shunt regulator as shown in FIG. 4. In thisway, current drawn from the telephone line will pass through the shuntregulator 400 as if the shunt regulator was not there, eliminating theneed to worry about designing a shunt regulator to draw only exactly apredetermined certain amount of current from a telephone line for use bya line modulator and other modem circuitry. All current flows in asingle series circuit as shown in FIG. 4, not in two parallel circuitsas shown in the common mode shunt regulator circuits shown in FIGS. 2and 3. Thus, there is no guessing in the shunt regulator 400 as toexactly how much current must be drawn from the telephone line for useby the modem circuitry. When the shunt regulator 400 is in series withcircuitry which it is powering, the shunt regulator knows exactly howmuch current is required from the telephone line.

However, unlike when a shunt regulator is placed in parallel with andthus in common mode with circuitry which it is powering as depicted inFIGS. 2 and 3, the series connection of a shunt regulator 400 and a linemodulator 402 introduces a noise problem in the DAA. The noise islargely a result of the line modulator 402 having a different groundreference than the shunt regulator 400. Thus, unless accounted for, thecircuit shown in FIG. 4 would exhibit a problem of undesired noise.

In accordance with the principles of the present invention, a feedbackloop is established to eliminate undesirable noise currents between theline modulator/DC termination circuit 402 and the shunt regulator 400,i.e., at node 460 shown in FIG. 4.

FIG. 5 shows a first embodiment of the present invention utilizing adifferential feedback loop to eliminate undesirable noise as sensedacross a sense resistor connected to the line power supply output of ashunt regulator.

In particular, FIG. 5 shows a shunt regulator 500 in series with a linemodulator/DC termination circuit 502. A sense resistor R_(sense) isplaced in series between the line modulator/DC termination circuit 502and the shunt regulator 500. In the first embodiment of the presentinvention, the nodes 570, 572 at both terminals of sense resistorR_(sense) provide a differential input to a differential feedbackcircuit 506 to reduce the noise otherwise present at node 570.

Differential feedback circuit 506 preferably has separate AC and DC gainproviding further controllable flexibility in the DAA for use incountries with different DC termination and AC modulation characteristicrequirements. The differential output of the differential feedbackcircuit 506 is converted to a single ended output by a differential tosingle ended converter 504. The single ended output is used to controlthe line modulator/DC termination circuit 502 in a way which allowsadjustment of the AC modulation and DC termination characteristicspresented to the telephone line.

An analog signal including an AC modulation component 508 and a DCoffset component 510 is input to the differential feedback circuit 506.The level of the DC offset component 510 controls the effective load ofthe DC termination presented to the telephone line by the linemodulator/DC termination circuit 502. Moreover, the AC signal 508 may beadjusted to change the AC characteristics of the output of the linemodulator/DC termination circuit 502 to correspond to desired ACcharacteristics peculiar to the standards of a particular country.

Accordingly, the DC termination and AC modulation presented by a DM to atelephone line are controlled in accordance with the principles of thepresent embodiment simply by adjusting a DC offset and AC signalprovided by a DSP or other device. In this way, only one deviceimplementing a DM according to the present invention need bemanufactured for virtually all countries, requiring only a softwarechange to adjust DC termination and/or AC modulation characteristics ofa DAA to allow use of such device in any of a plurality of countries.

FIG. 6 shows the DM of FIG. 5 in more detail.

In particular, the line modulator/DC termination circuit 502 comprises abipolar transistor 502 a in series with a sense resistor R_(sense) and ashunt regulator 500. A compensation capacitor 502 b is placed betweenthe base of the bipolar transistor 502 a and node 570.

The shunt regulator 500 includes an op amp 500 a driving a bipolartransistor 500 b to regulate the sourced power. The non-inverting inputof the op amp 500 a is connected to a voltage divider circuit comprisedof two series resistors 500 c, 500 d, and the inverting input of the opamp 500 a is set to a ground reference value BGREF. A storage capacitor500 e is placed between power and ground of the shunt regulator 500.

The differential feedback circuit 506 includes a differential output opamp 506 a. The non-inverting input of the differential op amp 506 a isconnected to node 572 via a parallel combination of a resistor 506 g,and a resistor 506 f and capacitor 506 j in series. The inverting inputof the differential op amp 506 a is connected to node 570 via a parallelcombination of a resistor 506 h, and a resistor 506 i and a capacitor506 k in series. The non-inverting output of the differential op amp 506a is fed back to the non-inverting input through a parallel combinationof a resistor 506 b and a capacitor 506 c. Similarly, the invertingoutput of the differential op amp 506 a is fed back to the invertinginput through a parallel combination of a resistor 506 d and capacitor506 e.

The differential output of the differential feedback circuit 506 isconverted to a single ended signal by the differential to single endedconverter 504. The differential signal is input to the inverting andnon-inverting inputs of a differential op amp 504 a through resistors504 b and 504 c, respectively. The positive node of the AC modulatingsignal 508 with DC offset 510 is input to the inverting input ofdifferential op amp 504 a through a resistor 504 d, while the negativenode of the AC modulating signal 508 with DC offset 510 is input to thenon-inverting input through a resistor 504 f. The inverting output ofdifferential op amp 504 a is fed back to its inverting input throughresistor 504 e, and the non-inverting output of differential op amp 504a is fed back to its non-inverting input through resistor 504 g. Thedifferential output of the differential op amp 504 a is converted to asingle ended signal at node 595 for controlling the line modulator/DCtermination circuit 502 by a parallel combination of MOS transistors 504h and 504 i, and bipolar transistor 504 j.

A bias voltage BIAS may be applied to the gate of a MOS transistor 504 kin series with the combination of transistors 504 h and 504 i, toprovide a current source for transistors 504 h and 504 i.

FIG. 7 shows an alternative embodiment utilizing a single ended feedbackloop and a common ground reference with the shunt regulator 700 toprovide controllable AC modulation and DC termination characteristics tothe telephone line.

In FIG. 7, only one terminal 703 of a sense resistor R_(sense) betweenthe series connection of the line modulator 702 and the shunt regulator700 is sensed to provide a single ended feedback loop. The circuit ofFIG. 7 is partially in series with the shunt regulator 700 yet retains acommon ground reference with the shunt regulator 700 to the ringpolarity signal RP.

FIG. 8 shows the circuit of FIG. 7 in more detail. In particular, theline modulator 702 includes two single ended feedback loops. Onefeedback loop is from tip polarity (TP), and feeds back DC line voltage.The other feedback loop is from one terminal 703 of the sense resistorR_(SENSE) and feeds back a voltage proportional to the line current.These feedback signals are used to set the DC termination and ACmodulation characteristics. The line voltage feedback is divided down byresistor 700 g and 700 h and fed to the non-inverting terminal of op amp700 a. Resistor 700 h is in parallel with capacitor 700 d to filter outAC voltage. The line current voltage feedback is divided down byresistor 700 f and resistor 700 e and fed to the inverting terminal ofop amp 700 a. The sense resistor R_(sense) is typically about 20 ohms,providing a gain of about thirty based on a telephone line impedance ofabout 600 ohms from the point of view of the tip polarity TP and ringpolarity RP terminals. Thus, the noise at node 703 is amplified about600/20=30 times as measured at tip polarity TP and ring polarity RP.

Of course, other values for the sense resistor R_(sense) may beappropriate, e.g., 10 ohms, based on the particular application and/orthe requirements of the central office.

Moreover, the shunt regulator 500, sense resistor R_(sense), and linemodulator/DC termination circuit 502 are in a series circuit between thetip polarity TP and ring polarity RP terminals. Accordingly, since thecurrent is equal through the series circuit, those of ordinary skill inthe art will appreciate that the order of the shunt regulator 500, senseresistor R_(sense), and line modulator/DC termination circuit 502 may beplaced in any order. For instance, the shunt regulator 500 as shown inFIG. 5 can be moved to a position between the tip polarity TP terminaland the line modulator/DC termination circuit 502 with the same resultsas described herein.

The shunt regulator 700 provides power drawn from the telephone line tothe line modulator/DC termination and feedback circuit 702 as well as toother modem and/or data processing circuitry necessary to provide a DCtermination and AC modulation to the telephone line. An AC signal 700 kfrom a DSP or other modem circuitry powered by the shunt regulator 702is AC coupled to the operational amplifier 700 a of the line modulator702 through a capacitor 700 i.

A bipolar pnp transistor 700 c placed in series with the shunt regulator700 provides the AC modulation to the telephone line, and draws theprescribed amount of current from the telephone line, i.e., presents aDC termination to the telephone line. The output of the operationalamplifier 700 a drives the base of bipolar npn transistor 700 b, thecollector of which drives the base of transistor 700 c. The tip polarityTP to ring polarity RP DC voltage is divided down and coupled to thenon-inverting terminal of operational amplifier 700 a through resistors700 g and 700 h and capacitor 700 d.

A portion of the DC termination of the line modulator/DCtermination/feedback circuit 702 is established by the value ofresistors 700 g, 700 h, 700 f and 700 e. Resistor 700 g is in serieswith resistor 700 h, and resistor 700 f is in series with resistor 700e, both series connections of which are placed in parallel with the tippolarity signal TP and ring polarity signal RP of the telephone line.

The circuit shown in FIGS. 7 and 8 senses the voltage V_(TP) withrespect to the ring polarity signal RP, and the current I_(TP) of thetip polarity signal TP, and feeds back proportional voltages V₁, V₂determined by the voltage division of resistors 700 f and 700 e, and 700g and 700 h, respectively, to the inverting and non-inverting inputs ofoperational amplifier 700 a, respectively. The AC component of thevoltage V_(TP) of the tip polarity signal TP is shunted to ground viacapacitor 700 d.

The governing equation for the equivalent impedance of the circuit shownin FIGS. 7 and 8 is as follows: $\begin{matrix}{{{Z_{in} = {\frac{V_{TP}}{I_{TP}} = {{R_{sense}\quad \frac{{s\quad C_{700d}R_{700g}R_{700h}} + R_{700g} + R_{700h}}{R_{700f} + R_{700e}}} + \frac{V_{Rsense}}{k_{1}I_{TP}}}}}{{where}\text{:}}}\quad} & {{Eq}.\quad (1)} \\{{k_{1} = {{\frac{R_{700h}}{R_{700e}}R_{700f}} + \frac{R_{700e}}{{s\quad C_{700d}R_{700g}R_{700h}} + R_{700g} + R_{700h}}}}{{{Equation}\quad (1)\quad {is}\quad {of}\quad {the}\quad {desired}\quad {form}\quad {sL}_{effective}} + {R_{effective}\quad {where}\text{:}}}} & {{Eq}.\quad (2)} \\{L_{effective} = \frac{C_{700d}R_{700g}R_{700h}R_{sense}}{R_{700f} + R_{700e}}} & {{Eq}.\quad (3)} \\{R_{effective} = {\frac{R_{sense}\left( {R_{700g} + R_{700h}} \right)}{R_{700f} + R_{700e}} + \frac{V_{SR}}{k_{1}I_{TP}}}} & {{Eq}.\quad (4)}\end{matrix}$

wherein V_(SR) represents the voltage across the shunt regulator. Theerror term in Equation (4), V_(SR)|k_(l)I_(TP), is to be considered whenthe component values for the circuit of FIGS. 7 and 8 are chosen.

The line modulation gain is determined (neglecting the impedance of ACcoupling capacitor C₇₀₀₁), by the following equation: $\begin{matrix}{A_{mod} = {\frac{R_{700f}}{R_{700j}}\frac{Z_{PSTN}}{R_{sense}}}} & {{Eq}.\quad (5)}\end{matrix}$

where Z_(PSTN) is the impedance of the public switched telephone network(PSTN) to which the telephone line is connected at the other end.

Accordingly, the present invention provides a low noise DAA particularlyuseful for modems including DSPs and/or other line powered devices, andwhich includes DC termination and AC modulation characteristics whichare software customizable for use in each of a plurality of countries.In this way, a common DAA can be manufactured and used in products inmany countries, with the only difference between the products used inthe various countries being in the software.

While the invention has been described with reference to the exemplarypreferred embodiments thereof, those skilled in the art will be able tomake various modifications to the described embodiments of the inventionwithout departing from the true spirit and scope of the invention.

We claim:
 1. A telephone line interface comprising: a shunt regulator; aline modulator in series with said shunt regulator between a tip sideand a ring side of a telephone line input, to maximize line power withreduced noise; an amplifier, powered by said shunt regulator, to drivesaid line modulator; and a feedback loop from said line modulator tosaid amplifier.
 2. The telephone line interface according to claim 1,further comprising: a sense resistor connected to said shunt regulator.3. The telephone line interface according to claim 2, wherein: saidsense resistor is connected in series between said shunt regulator andsaid line modulator.
 4. The telephone line interface according to claim2, wherein: said feedback loop is connected to said sense resistor. 5.The telephone line interface according to claim 4, wherein: saidfeedback loop is a differential feedback loop.
 6. The telephone lineinterface according to claim 5, wherein: said differential feedback loopis connected to opposite ends of said sense resistor.
 7. The telephoneline interface according to claim 1, wherein said feedback loopincludes: a differential to single ended converter.
 8. The telephoneline interface according to claim 7, further comprising: a modulated ACsignal having a DC offset voltage; said modulated AC signal having saidDC offset voltage being input to said differential to single endedconverter.
 9. The telephone line interface according to claim 1,wherein: said telephone line interface is a data access arrangement fora modem.
 10. A telephone line interface comprising: a shunt regulator todraw power from a telephone line; a sense resistor connected in serieswith said shunt regulator; a line modulator in series with said senseresistor and said shunt regulator between a tip side and a ring side ofa telephone line input, to maximize telephone line power with reducednoise; and a differential feedback path from opposite terminals of saidsense resistor to said line modulator.
 11. The telephone line interfaceaccording to claim 10, further comprising: a differential to singleended converter to convert said differential feedback path to a singleended control path to modulate said line modulator.
 12. A method ofpowering customer premises equipment from a telephone line, comprising:drawing power from said telephone line with a shunt regulator; providinga line modulator in series with said shunt regulator between a tip sideand a ring side of a telephone line input, to maximize telephone linepower with reduced noise; powering an amplifier with power drawn fromsaid telephone line; driving said line modulator with said line poweredamplifier; and feeding back a signal from said line modulator to aninput of said line powered amplifier.
 13. The method of poweringcustomer premises equipment according to claim 12, further comprising:differentially sensing an amount of noise in a telephone line interfaceof said customer premises equipment.
 14. The method of powering customerpremises equipment according to claim 13, wherein: said signal fed backfrom said line modulator is representative of said sensed amount ofnoise.
 15. The method of powering customer premises equipment accordingto claim 13, wherein: said amount of noise is sensed across a senseresistor.
 16. The method of powering customer premises equipmentaccording to claim 13, wherein: said amount of noise is differentiallysensed from two points between said line modulator and said shuntregulator.
 17. Apparatus for powering customer premises equipment from atelephone line, comprising: means for drawing power from said telephoneline; means for modulating said telephone line, connected in series withsaid means for drawing power from said telephone line between a tip sideand a ring side of a telephone line input, said means for modulatingsaid telephone line maximizing telephone line power with reduced noise;an amplifier powered by said means for drawing power from said telephoneline; means for driving said means for modulating said telephone line;and means for feeding back a signal from said means for modulating to aninput of said amplifier.
 18. The apparatus for powering customerpremises equipment according to claim 17, wherein said means for feedingback said signal includes: means for differentially sensing an amount ofnoise in a telephone line interface of said customer premises equipment.19. The apparatus for powering customer premises equipment according toclaim 18, further comprising: noise sensing means connected between saidmeans for modulating and said means for drawing power from saidtelephone line.
 20. The apparatus for powering customer premisesequipment according to claim 18, wherein said means for differentiallysensing includes: a sense resistor connected between said means fordrawing powering and said means for modulating.